Not applicable.
Not applicable.
The present invention relates to an apparatus and a method for refining and casting metal and metal alloy ingots and other preforms. The present invention more particularly relates to an apparatus and a method useful for refining and casting large diameter ingots and other preforms of metals and metal alloys prone to segregation during casting, and wherein the preforms formed by the apparatus and method may exhibit minimal segregation and lack significant melt-related defects. The apparatus and method of the invention find particular application in, for example, the refinement and casting of complex nickel-based superalloys, such as alloy 706 and alloy 718, as well as certain titanium alloys, steels, and cobalt-base alloys that are prone to segregation when cast by conventional, state-of-the-art methods. The present invention is also directed to preforms and other articles produced by the method and/or apparatus of the present invention.
In certain critical applications, components must be manufactured from large diameter metal or metal alloy preforms exhibiting minimal segregation and which are substantially free of melt-related defects such as white spots and freckles. (For ease of reference, the term xe2x80x9cmetallic materialxe2x80x9d is used herein to refer collectively to unalloyed metals and to metal alloys.) These critical applications include use of metal components as rotating components in aeronautical or land-based turbines and in other applications in which metallurgical defects may result in catastrophic failure of the component. So that preforms from which these components are produced are free of deleterious non-metallic inclusions, the molten metallic material must be appropriately cleaned or refined before being cast into a preform. If the metallic materials used in such applications are prone to segregation when cast, they are typically refined by a xe2x80x9ctriple meltxe2x80x9d technique which combines, sequentially, vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). Metallic materials prone to segregation, however, are difficult to produce in large diameters by VAR melting, the last step in the triple melt sequence, because it is difficult to achieve a cooling rate that is sufficient to minimize segregation. Although solidification microsegregation can be minimized by subjecting cast ingots to lengthy homogenization treatments, such treatments are not totally effective and may be costly. In addition, VAR often will introduce macro-scale defects, such as white spots, freckles, center segregation, etc., into the ingots. In some cases, large diameter ingots are fabricated into single components, so VAR-introduced defects cannot be selectively removed prior to component fabrication. Consequently, the entire ingot or a portion of the ingot may need to be scrapped. Thus, disadvantages of the triple melt technique may include large yield losses, lengthy cycle times, high materials processing costs, and the inability to produce large-sized ingots of segregation-prone metallic materials of acceptable metallurgical quality.
One known method for producing high quality preforms from melts of segregation prone metallic materials is spray forming, which is generally described in, for example, U.S. Pat. Nos. 5,325,906 and 5,348,566. Spray forming is essentially a xe2x80x9cmoldlessxe2x80x9d process using gas atomization to create a spray of droplets of liquid metal from a stream of molten metal. The process parameters of the spray forming technique are adjusted such that the average fraction of solid within the atomized droplets at the instant of impact with a collector surface is sufficiently high to yield a high viscosity deposit capable of assuming and maintaining a desired geometry. High gas-to-metal mass ratios (one or greater) are required to maintain the heat balance critical to proper solidification of the preform.
Spray forming suffers from a number of disadvantages that make its application to the formation of large diameter preforms problematic. An unavoidable byproduct of spray forming is overspray, wherein the metal misses the developing preform altogether or solidifies in flight without attaching to the preform. Average yield losses due to overspray in spray forming can be 20-30%. Also, because relatively high gas-to-metal ratios are required to maintain the critical heat balance necessary to produce the appropriate solids fraction within the droplets on impact with the collector or developing preform, the rapid solidification of the material following impact tends to entrap the atomizing gas, resulting in the formation of gas pores within the preform.
A significant limitation of spray forming preforms from segregation prone metallic materials is that preforms of only limited maximum diameter can be formed without adversely affecting microstructure and macrostructure. Producing larger spray formed preforms of acceptable quality requires increasingly greater control of the local temperature of the spray to ensure that a semi-liquid spray surface layer is maintained at all times. For example, a relatively cooler spray may be desirable near the center of the preform, while a progressively warmer spray is desired as the spray approaches the outer, quicker cooling areas of the preform. The effective maximum diameter of the preform is also limited by the physics of the spray forming process. With a single nozzle, the largest preforms possible have a maximum diameter of approximately 12-14 inches. This size limitation has been established empirically due to the fact that as the diameter of the preform increases, the rotational speed of the surface of the preform increases, increasing the centrifugal force experienced at the semi-liquid layer. As the diameter of the preform approaches the 12 inch range, the increased centrifugal force exerted on the semi-liquid layer tends to cause the layer to be thrown from the preform face.
Accordingly, there are significant drawbacks associated with certain known techniques applied in the refining and casting of preforms, particularly large diameter preforms, from segregation prone metallic materials. Thus, a need exists for an improved apparatus and method for refining and casting segregation prone metals and metal alloys.
In order to address the above-described need, the present invention provides a method of refining and casting a preform including the steps of providing a consumable electrode of a metallic material and then melting and refining the electrode to provide a molten refined material. At least a portion of the molten refined material passes through a passage that is protected from contamination by contact with oxygen in the ambient air. The passage preferably is constructed of a material that will not react with the molten refined material. A droplet spray of the molten refined material is formed by impinging a gas on a flow of the molten refined material emerging from the passage. The droplet spray is deposited within a mold and solidified to a preform. The preform may be processed to provide a desired article such as, for example, a component adapted for rotation in an aeronautical or land-based turbine.
The step of melting and refining the consumable electrode may consist of at least one of electroslag remelting the consumable electrode and vacuum arc remelting the consumable electrode to provide the molten refined material. The passage through which the molten refined material then passes may be a passage formed through a cold induction guide. At least a portion of the molten refined alloy passes through the cold induction guide and is inductively heated within the passage. In less demanding applications, e.g., applications in which some small level of oxide contaminants in the alloy can be tolerated, a cold induction guide need not be used. Components used in such less demanding applications include, for example, static components of aircraft turbine engines. In cases in which a cold induction guide is not used, the passage may be an unheated passage protected from the atmosphere and including walls composed of a refractory material. The passage may be adapted to protect the molten refined material from undesirable impurities. The molten refined material emerging from the passage is then solidified to a preform as noted above.
The present invention also addresses the above-described need by providing an apparatus for refining and casting an alloy. The apparatus includes a melting and refining apparatus that includes: at least one of an electroslag remelting apparatus and a vacuum arc remelting apparatus; a transfer apparatus (such as, for example, a cold induction guide) in fluid communication with the melting and refining apparatus; and a nucleated casting apparatus in fluid communication with the transfer apparatus. A consumable electrode of a metallic material introduced into the melting and refining apparatus is melted and refined, and the molten refined material passes to the nucleated casting apparatus via a passage formed through the transfer apparatus. In the case where the transfer apparatus is a cold induction guide, at least a portion of the refined material is retained in molten form in the passage of the cold induction guide by inductive heating.
When casting a metallic material by certain embodiments of the method of the present invention, the material need not contact the oxide refractories used in the melting crucibles and pouring nozzles utilized in conventional casting processes. Thus, the oxide contamination that occurs on spalling, erosion, and reaction of such refractory materials may be avoided.
The electroslag remelting apparatus that may be a part of the refining and casting apparatus of the present invention includes a vessel having an aperture therein, an electric power supply in contact with the vessel, and an electrode feed mechanism configured to advance a consumable electrode into the vessel as material is melted from the electrode during the electroslag remelting procedure. A vacuum arc remelting apparatus differs from an electroslag remelting apparatus in that the consumable electrode is melted in a vessel by means of a DC arc under partial vacuum, and the molten alloy droplets pass to the transfer apparatus of the apparatus of the invention without first contacting a slag. Although vacuum arc remelting does not remove microscale inclusions to the extent of electroslag remelting, it has the advantages of removing dissolved gases and minimizing high vapor pressure trace elements in the electrode material.
The cold induction guide that may be a part of the casting and refining apparatus of the invention generally includes a melt collection region that is in direct or indirect fluid communication with the aperture of the vessel of the melting and refining apparatus. The cold induction guide also includes a transfer region defining the passage, which terminates in an orifice. At least one electrically conductive coil may be associated with the transfer region and may be used to inductively heat material passing through the passage. One or more coolant circulation passages also may be associated with the transfer region to allow for cooling of the inductive coils and the adjacent wall of the passage.
The nucleated casting apparatus of the casting and refining apparatus of the invention includes an atomizing nozzle in direct or indirect fluid communication with the passage of the transfer apparatus. An atomizing gas supply is in communication with the nozzle and forms a droplet spray from a flow of a melt received from the transfer apparatus. A mold, which includes a base and side wall to which the preform conforms, is disposed adjacent to the atomizing nozzle, and the position of the mold base relative to the atomizing nozzle may be adjustable.
The method and apparatus of the invention allow a refined melt of a metallic material to be transferred to the nucleated casting apparatus in molten or semi-molten form and with a substantially reduced possibility of recontamination of the melt by oxide or solid impurities. The nucleated casting technique allows for the formation of fine grained preforms lacking substantial segregation and melt-related defects associated with other casting methods. By associating the refining and casting features of the invention via the transfer apparatus, large or multiple consumable electrodes may be electroslag remelted or vacuum arc remelted to form a continuous stream of refined molten material that is nucleation cast into a fine grained preform. In that way, preforms of large diameter may be conveniently cast from metallic materials prone to segregation or that are otherwise difficult to cast by other methods. Conducting the method of the invention using large and/or consumable electrodes also makes it possible to cast large preforms in a continuous manner.
Accordingly, the present invention also is directed to preforms produced by the method and/or apparatus of the invention, as well as articles such as, for example, components for aeronautical or land-based turbines, produced by processing the preforms of the present invention. The present invention also is directed to preforms and ingots of segregation prone alloys of 12 inches or more in diameter and which lack significant melt-related defects. Such preforms and ingots of the invention may be produced by the method and apparatus of the present invention with levels of segregation characteristic of smaller diameter VAR or ESR ingots of the same material. Such segregation prone alloys include, for example, alloy 706, alloy 718, alloy 720, Rene 88, and other nickel-based superalloys.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend such additional advantages and details of the present invention upon carrying out or using the invention.